Harmonic generator



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Filed Nov. so, 194e V. C. WESTCOTT HARNONIC GENERATOR iff/6:

2 Sheets-Sheet 1 @Mig l 'A 2 Sheets-Sheet 2 V. C. WESTCOTT HARMONICGENERATOR Oct. 18, 1949.

Filed Nov. so, 1946 phpph VERA/0A 6I wfsrcarr Patentecl Oct. 18, 1949HARMONIC GENERATOR Vernon C. Westcott, Lincoln, Mass., assignor toRaytheon Manufacturing Company, Newton, Mass., a corporation of DelawareApplication November 30, 1946, Serial No. 713,251

(Cl. Z50-36) 1 Claim.

This invention relates to electrical circuits and more particularly tocircuits for producing a plurality of frequencies having and maintaininga predetermined ratio therebetween.

The main object of the present invention is to provide an electricalcircuit capable of producing a plurality of frequencies which areharmonics of a fundamental frequency and maintaining between saidharmonic frequencies a predetermined frequency ratio.

The above and other objects will become clear as the description of thepresent invention progresses and by reference to the accompanyingdrawing, in which:

Fig. 1 is a partial schematic and partial block diagram of the presentinvention; and

Fig. 2 is a schematic diagram of another embodiment of the presentinvention.

Referring now more particularly to Fig. 1, an electron-discharge tube,for example a tetrode, is shown at Ill, said tube having an anode ascreen grid I2, a control grid |3, and a cathode It, said cathode beingheated by suitable means (not shown) to produce thermionic emission.

Screen grid I2 is connected to a source of potential, for example abattery l5, through a current-limiting resistor I6, and is alsoconnected to ground Il through a capacitor I8 and directly to a screengrid I9 of a second electron-discharge tube 2B, said tube being, forexample, a tetrode having, in addition to the screen grid, a controlgrid 2|, a cathode 22, said cathode being suitably heated to producethermionic emission, and an anode 23.

The cathodes |'4 and 22 are connected through a self-biasingarrangement, comprising, respectively, resistors 24 and 25, to theground the ungrounded ends of said resistors being connected,respectively, to output terminals 26 and 28, terminals 21 and 29 beingconnected to th ground point The anodes and 23 are connected in parallelby a conductor 3U and through a delay line 3| to the potential source|5. The line 3| consists of a series inductance 32 having a movable tap33 thereon, and is so constructed as to provide distributed shuntcapacitance 34 therealong. This line is terminated in its characteristicimpedance as represented by a resistor 35, whereby` reflections or wavestravelling along said line are prevented.

Tap 33 is connected to one end of a potentiometer 36 having a movabletap 31 thereon, the other end of said potentiometer being connected toground Il. The tap 31 is connected through a pair of -band-pass lters 38and 39 to the respective grids |3 and 2|. The choice of a filter isdetermined by the particular frequency desired.

To produce oscillations in a tube circuit, it is essential that there bea phase shift of 360 degrees around the system, from the input to theoutput and back to the input again, and it is also essential that thegain around the system be at least equal to unity. If these conditionsare fulfilled, the system will oscillate at a frequency for which thereis aphase shift of 360 degrees around the system, if the system gain forsuch a frequency is at least equal to unity.

There is ordinarily a phase difference of degrees between grid and anodepotentials due to the inherent properties of an electron-discharge tube,so that if an additional phase shift of 180 degrees can be imparted to awave by an external system, the required 360 degree shift will beprocured.

Transmission or delay line 3|, due to its inductance and capacitance,will impart a phase shift or phase delay to waves travelling therealong.For any fixed position of tap 33, there is a certain frequency for whichthe phase delay between anodes and 23 and tap 33 will be substantially180 degrees, and the circuit will oscillate at this certain frequency.In other words, for any flxed position of tap 33, there is a certainfrequency at which the time required for a wave to travel from theanodes and 23 to the tap 33, will be substantially equal to onehalfcycle of this frequency, or 180 degrees, and

' the system will oscillate at this frequency.

The frequency of oscillation of the system may be varied by moving tap33, since for each different position of said tap there is a certaindifferent frequency at which the proper phase relations for oscillationoccur. As it is possible to make the transmission line as long asdesired, and since it is theoretically possible to bring tap 33infinitely close to anodes and 23, a broadband oscillator is provided.Therefore, a tremendous frequency range, such as, for example, from 30-kilocycles to 20 megacycles, may be obtained with a single oscillator.

As indicated above, for any fixed position of tap 33 there exists acertain fundamental frequency and all odd harmonics of that fundamental.Thus, there may be obtained various wave forms, from a pure sinusoidalto a square wave. The delay line 3| attenuates the lower frequenciesless than the higher ones, While the potentiometer 36 attenuates allfrequencies equally. Therefore, by moving tap 31 on said potentiometertoward the grounded end |1 thereof, the system gain may be made equal tounity for the fundamental frequency and less than unity for the oddharmonics, in which case the odd harmonies will not appear in theoscillator output and a pure sine wave will be produced.

The closer the tap 31 is to the delay line end of the potentiometer 36,the more harmonics will be mixed with the fundamental and the closer toa square wave the oscillator output will be. Therefore, the wave form ofthe oscillator output may be varied for any particular fundamentaldetermined by the position of the tap 33 and the adjustment of the tap31.

In the present invention, the output of the above-described oscillatoris applied to the filters 38 and 39 through the potentiometer 36 and itsassociated tap 31. The lters 38 and 39 are designed to pass,respectively, different predetermined frequencies. In this particularembodiment the use of two filters are described, although any number offilters may be used. The predetermined frequencies are multiples orharmonics of the fundamental frequency of the delay line 3| oscillatoras selected by manipulation of the taps 33 and 31.

The outputs of the filters 38 and 39 are applied, respectively, to thecontrol grids I3 and 2| of the electron-discharge tubes I and 20. Theparticular frequencies selected are impressed across the bias resistors24 and 25 and consequently appear between the terminals 26, 21, 28 and29.

The invention, therefore, provides a means for deriving any number ofdifferent frequencies which are multiples of the fundamental frequencyX. For example, the filters 38 and 39 are designed to pass frequencies,respectively, three times the fundamental frequency X and five timessaid fundamental. Actually, the present invention provides means forderiving a substantially large number of frequencies depending on thefundamental selected by manipulation of the taps 33 and 31.

Ordinarily, multiplication obtained by energizing a resonant frequencycircuit, tuned to some multiple of the fundamental frequency of anoscillator, is limited to two or three times said fundamental. However,the present invention is capable of producing a substantially largenumber of multiples with the present system because of the extremelywide frequency range of the delay line oscillator used herein.

In addition, the system of the present invention is such that for anyselected number of different frequencies, said frequencies aremaintained in a fixed ratio to one another. For example, the presentembodiment describes the selection of two frequencies which are to bemaintained at a three to live ratio. It is only necessary to determinethe ratio desired, within the harmonic limits of the fundamentalfrequency fX, and by proper selection of filters 38 and 39 provide thedesired ratio.

Thereafter, with any drift from the selected multiple of the fundamentalfrequency, as in the present embodiment, BfX or 5fX, the system of thepresent invention will cause either one or the other of said multiplesto synchronize with one or the other to maintain said three to ve ratio.As the complex wave form, comprising 3]X and 5fX, goes through the delayline 3|, it is either advanced or retarded, as the case may be, and thusinfluences both tubes simultaneously.

Fig. 2 represents another embodiment of the present invention. Here, aplurality of amplifying stages 46, 41 and 48 have been added,respectively, to a plurality of frequency-selecting circuits 49, 59 and5l. The outputs of said frequency-selecting circuits are connected,respectively, through a plurality of capacitors 52, 53 and 54 to thecontrol grids 55, 56 and 51 of a plurality of electron-discharge tubes58, 59 and 60. The

selected frequencies will appear across a plurality y of pairs ofterminals 6|, 62 and 63.

Selection of a fundamental frequency is accomplished in this particularembodiment in the same manner as hereinbefore described, namely, throughmanipulation of the taps 33 and 31.

The present invention, although illustrated and described in terms oflumped constants, is readily adapted for use in connection withultra-high frequency systems wherein are employed wave guides, cavityresonators, etc.

Many modifications of the present invention will occur to those skilledin the art, and it is therefore, intended to cover by the appended claimall such modifications which fall within the true spirit and scope ofsaid invention.

What is claimed is: y

An electrical circuit including: an electrical delay line having avariable fundamental frequency characteristic; a plurality ofelectron-discharge devices having at least input and output electrodes;said output electrodes being connected to one end of said delay line; aplurality of frequency-selective circuits responsive, respectively, todifferent harmonics of any selected fundamental frequency of said delayline; means controlling the harmonic content of the output of said delayline and connecting said frequencyselective circuits between the otherend of said delay line and the input electrodes of saidelectron-discharge devices; and means for deriving from saidelectron-discharge devices a plurality of harmonics having apredetermined frequency ratio therebetween.

VERNON C. WESTCOTT.

REFERENCES CITED The following references are of record in the le ofthis patent:

UNITED STATES PA'I'ENTS Number Name Date 1,418,739 Schriven June 6, 19221,442,781 Nichols Jan. 16, 1923 1,678,199 Rettenmeyer July 24, 19282,036,495 Pohontsch Apr. 7, 1936

